THank you for the hints and discussion. I think u all are right....bd_claim and O_EXCL is the answer.

First, when u open a device with O_EXCL flag, u lock the buffer as the API blkdev_open() shows:

static int blkdev_open(struct inode * inode, struct file * filp)
{
struct block_device *bdev;
int res;

/*
* Preserve backwards compatibility and allow large file access
* even if userspace doesn't ask for it explicitly. Some mkfs
* binary needs it. We might want to drop this workaround
* during an unstable branch.
*/
filp->f_flags |= O_LARGEFILE;

bdev = bd_acquire(inode);
if (bdev == NULL)
return -ENOMEM;

res = do_open(bdev, filp, 0);
if (res)
return res;

if (!(filp->f_flags & O_EXCL) )
return 0;

if (!(res = bd_claim(bdev, filp)))==================> buffer is locked, at the VFS level.
return 0;

blkdev_put(bdev);
return res;
}

Since it worked at the VFS level, it is the same for all the different filesystem like ext3 or ext4 etc.

There goes your userspace solution as well.